It’s not intended to break if another MailChimp plugin is activated, but another MailChimp plugin activated on the same site could introduce a plugin conflict that would cause some breakage. Especially so if the other newsletter form was on the same page as the event registration form. Is that the case on your site?

One thing that might avoid a conflict with the other MailChimp plugin is you could remove the Newsletter signup widget from the /event-registration/ page only. You could install a plugin like Widget Context, then go to edit the Newsletter signup widget and set the option to hide it from the event-registration page.